The use of time gain control (TGC) in medical ultrasonic imaging and non-destructive testing is well known. In ultrasonic imaging, pulses of acoustic energy reflected from or transmitted though an object are measured and processed to produce images of the object. As an ultrasonic pulse propagates through an object it is generally subject to attenuation due to a variety of physical mechanisms. Generally, the amount of attenuation increases with the length (in time or distance) the propagation path. Thus, for example, a reflection from impedance mismatch deep in an object is generally at a lower level than a reflection from a similar impedance mismatch nearer the surface.
Measurement systems used to capture the reflected or transmitted sound are imperfect. In particular, they have limited dynamic range and limited resolution.
The effect of limited dynamic range may be mitigated by using non-linear signal compression to boost low level signals relative to high level signals before the signals are captured.
A further approach is to use a variable gain amplifier to increase the gain of the signal path over time, as the signal level decreases. This approach has been used successfully in ultrasound systems for imaging bulk materials. In this application, the decrease of the signal strength with time is largely due to geometric spreading and absorption in the material. Thus, the gain of the variable gain amplifier is generally increased with time in a simple geometric and/or exponential manner.
This approach has also been used successfully in medical ultrasound. In medical ultrasound, an image is captured using arrays of transmitting and receiving transducers, together with phased-array beamforming.
The structure of the part of the body being scanned is complicated and three-dimensional, so again the gain of the variable gain amplifier is generally increased with time in simple geometric or exponential manner defined by a single parameter that may be adjusted by an operator. For example, the operator may view repeated C-scan displays, adjusting the parameter between scans to obtain an image that appears to be improved.
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